In situations where sound systems are being used in noisy environments, the audibility of the sound system often suffers from ambient noise level fluctuations due to change in the activity level with the room environment. One common solution to this problem is for an individual to adjust the volume of the sound system according to a subjective evaluation of the level of ambient noise in the environment. This approach works well when the information content of the broadcast is low, as is often the case in background music systems. However, in foreground systems where announcements or commercials are being broadcast, the information content is higher and it is of paramount importance for individuals to hear the program material regardless of background noise level. Human control of the volume of the sound system is generally inadequate to insure proper control of a sound system for use in such a foregoing information system. Furthermore, human control is generally inadequate in environments where the ambient noise may change by a significant amount during the period of a day.
One solution to the above described problems is the use of an "automatic gain control" (AGC) device to maintain the volume of the sound system at the desired level. In theory, AGCs maintain constant audibility by automatically adjusting the sound level to compensate for the daily fluctuations of ambient noise in the environment. One of the common problems in prior art AGCs, however, is a phenomenon termed "runaway," a condition where the sound volume increases to a level that is high enough to become intolerable. The most significant contributing factor to runaway is the dependence of prior art AGCs on signal level and bandwidth.
Many prior art techniques have been proposed for automatically adjusting the volume of a sound system according to the level of ambient noise in the environment. In U.S. Pat. No. 3,290,442, a technique is described whereby a portion of the signal input to the loudspeaker is used in conjunction with a signal derived from a microphone channel to maintain a sound level that is a constant ratio to the noise volume. Mathematical analysis of this particular technique, however, indicates that the gain adjusting effect of the circuit is dependent upon the signal itself, thus indicating the creating of a runaway condition under certain circumstances of signal and noise. In another approach, U.S. Pat. Nos. 3,934,084 and 3,934,085 describe a technique whereby the absence of an input signal is detected and, during the interval of silence, the ambient noise is sampled by a microphone channel. The gain of the input signal channel is increased or decreased according to the resulting noise measurement in preparation for the next input signal.
Other techniques for automatic gain control, such as those disclosed in U.S. Pat. Nos. 4,247,955; 4,476,571; 4,553,257; and 4,641,344 provide various means for overcoming shortfalls associated with car radio and stereo systems with regard to varying ambient noise conditions. In most of these techniques, it is assumed that the noise is sampled in a frequency band that is separated from the information band either naturally or with the help or filtering, so that the operation of the circuit is not affected by the signal characteristics.
Another patent, U.S. Pat. No. 4,479,237 provides a means for a variable slope control, whereby the sound level is increased as a function of ambient noise, depending upon the magnitude of the means noise level. Finally, another technique is described in U.S. Pat. No. 4,628,526 in which noise and signal are both detected in a microphone channel and then compared to the signal channel. The degree of similarity is then derived by forming comparisons of the slopes of the envelopes of the two channels. The degree of similarity is used to determine the amount of ambient noise present, which leads to an automatic adjustment of the signal volume.
To operate properly, an AGC should be designed to maintain a constant power ratio of signal to noise. The AGC provided by the present invention, as described in greater detail below, advances the state of the art by providing an automatic gain control system which is capable of maintaining a constant power ratio of signal to noise, thus maintaining audible levels of program material without being susceptible to runaway.